1. Field of the Invention
The present invention relates to a thin film forming apparatus and, more specifically, to a thin film forming apparatus which forms a thin film over the surface of a substrate by spraying an atomized source solution over the surface of the heated substrate.
2. Description of the Prior Art
A conventional thin film forming apparatus uses a spraying system spray a source solution prepared beforehand over the surface of a substrate to form a thin film over the surface of the substrate. The thin film forming apparatus is provided with an atomizer for atomizing a previously prepared source solution, a nozzle disposed above the atomizer to spout the atomized source solution into a film forming chamber, and an exhaust duct connected to the film forming chamber. The atomizer comprises a mist box and a sprayer disposed in the mist box. Only a mist of comparatively small droplets of the source solution is spouted through the nozzle into the film forming chamber, and then the mist is exhausted from the film forming chamber through the exhaust duct.
Substrates arranged successively in a line are moved at a given speed from a preheating chamber through the film forming chamber to an exit. While being moved through the preheating chamber, the film forming chamber and the exit, the substrates are heated with a heater placed opposite to the backsides of the substrates. While the mist of the source solution flows through the film forming chamber along the surfaces of the heated substrates, a material contained in the solution reacts with oxygen contained the air or the moisture of the source solution and a generated oxide deposits on the surfaces of the substrates as a thin film.
This conventional thin film forming apparatus supports and guides the substrate at its opposite side edges by the side walls of the film forming chamber. The distance between the side walls of the film forming chamber corresponds to the width of the substrate. Therefore, the thin film forming apparatus is unable to process substrates having a width different from a particular width corresponding to the distance between the side walls of the film forming chamber.
The conventional thin film forming apparatus has a drawback in forming a transparent conductive film over the surface of a substrate, such as a glass substrate in that the thickness of the transparent conductive film is very small at the peripheral portions of the substrate as compared with the central portion as shown in FIG. 5(b). Such a drawback is inferred to be due to the fact that the flow rate per unit flow passage area of the mist in the vicinity of the opposite side edges of the substrate, particularly in the rear portion of the film forming chamber, is reduced by the resistance of the side walls and hence the flow rate of the mist in the vicinity of the side edges of the substrate is smaller than that of the mist in the central portion of the substrate.
Such a transparent conductive film having an irregular thickness forms interference fringes in the opposite sides of the substrate. Such a transparent, conductive film is unsatisfactory in appearance, and the thinner portions of the transparent, conductive film formed in the opposite sides of the substrate have no required characteristics. Accordingly, the opposite side portions of the substrate, where the thickness of the transparent, conductive film is excessively small, must be removed. For example, when a tin oxide film is formed over the surface of a glass substrate by a conventional thin film forming apparatus, the width of the side portions in which the difference of the thickness of the tin oxide film from the thickness of the tin oxide film formed in the central portion of the substrate is .+-.5% is as large as 30% of the width of the effective film forming area of the substrate excluding the opposite side edges of the substrate supported on the side walls. Therefore, practical width of the usable area of the substrate is as small as 70% of the width of the effective forming area. Thus, the conventional thin film forming apparatus is unsatisfactory.